1. Field of the Invention
The present invention relates to an autofocus camera wherein a photographing lens is driven on the basis of an output from a range finding device, and to a method of focus control therefor.
2. Description of the Related Art
In a camera having an autofocusing mechanism, a large time lag occurs between the time point of shutter release and the start of exposure. Consequently, when an object moving along the optical axis of the camera is to be photographed, a photographing lens may not be correctly focused.
Techniques for solving this problem are disclosed in Published Unexamined Japanese Patent Application (PUJPA) No. 63-118133 and PUJPA No. 63-159817. According to these techniques, the velocity of an object is detected and the position of the object at the time of exposure is determined in advance, thereby driving the photographing lens. Specifically, on the basis of the distance detected at the first time point and the distance detected at the second time point (a predetermined time after the first time point), the velocity of the object is calculated.
However, according to the technique of PUJPA No. 63-159817 wherein complex calculation is performed to find the distance of the object and thereafter the lens is driven, the release time lag (calculation time+time required for driving the lens+time for starting the shutter operation) increases and the best chance for releasing the shutter may be missed.
Suppose that the time required for finding the amount of movement of the object is 10 msec. and the release time lag is 400 msec. In this case, on the basis of the amount of movement of the object during 10 msec., the distance from the camera to the object (hereinafter called "object distance") at the time point of exposure start after 400 msec is estimated. More specifically, the error in velocity of the object is increased 40 times, and the possibility of defocusing becomes higher in such a range finding device as an active triangular range finding device with a high error in distance found. If the time required for measuring the amount of movement of the object is increased to reduce the error, the time lag increases further and the chance for releasing the shutter may be missed.